You know exactly, that Alexa means really NOTHING! - it is only for Alexa Tool Bar User - and that is not so much!! - do you have installed Alexa? What is Alexa Ranking?
This is a ranking system set by alexa.com (a subsidiary of amazon.com) that basically audits and makes public the frequency of visits on various Web sites. The algorithm according to which Alexa traffic ranking is calculated, is simple. It is based on the amount of traffic recorded from users that have the Alexa toolbar installed over a period of three months.
Not everybody has the Alexa toolbar installed, so there might be millions of Web sites that, even if they have a lot of traffic, will not be ranked (or not high enough) by Alexa. It is rather relative.
Many people say that it is inaccurate and that Alexa traffic can be greatly influenced (or "gamed", as some prefer to call it).
Subdomains are not ranked separately, and neither are subpages within a domain. The overall traffic is calculated for the top-level domain only.